The blade geometry of a conventional screwdriver has oppositely facing planar blade surfaces that converge to a tip with an included angle of about 10.degree.. This construction causes the screwdriver to cam out of the screw slot when rotational force is applied to screws resistant to turning. In an effort to minimize this tendency, the application of increased axial pressure in the direction of the screw is necessary as torque is being applied. In addition, the blade geometry creates point contact between the screwdriver and the screw slot and this together with the camming effect frequently causes deformation of the screw slot and an impediment to reuse of the screw.
Specially designed screwdrivers have previously incorporated oppositely facing parallel or diverging blade tip surfaces. Although these screwdrivers maintain screw head contact with less axial pressure applied than is necessary with the conventional blade construction, the tips of such screwdrivers have always been so thin and thus structurally weak as to be susceptible to breakage. This is a particular problem when such screwdrivers are used as prys or chisels, as often is done.
Specially designed screwdriver-screw head systems facilitate screwdriver-screw engagement with less axial pressure than conventional screwdrivers and screw heads. However, a system that requires a screw head to be shaped to accommodate a particular screwdriver is not readily acceptable by consumers and industry.
Prior art patent noted by the investigation conducted for the present invention are described below.
U.S. Pat. No. 67,014 to Ayres for Screwdriver discloses a screw driver having a dove-tailed edge on its tip to facilitate screw head engagement.
U.S. Pat. No. 260,795 to Smith for Screw discloses a screw having a projection in the center of the screw slot and a screwdriver having a notch in the tip to hold the screwdriver in the screw slot.
U.S. Pat. No. 417,722 to Hart for Screw Driver discloses a screw driver having sunk portions at the opposite sides of the blade which engage the screw slot.
U.S. Pat. No. 432,928 to Bartlett discloses a Wrench for Screw Drivers which is used to increase the torque that can be applied during use.
U.S. Pat. No. 1,056,095 to Groos for Screw Driver discloses a screw driver having two transverse grooves on each surface of the screw driver tip to engage corresponding portions of the undercut groove in the screw head.
U.S. Pat. No. 1,361,790 to Brown for Valve And Grinding Mechanism Therefor discloses a valve and a valve grinding tool. The Brown valve includes a groove therein that has a curved bottom and side walls that include toward each other from the bottom of the groove to the top of the valve. The Brown valve grinding tool includes an integral tongue shaped in conformity with and adapted to fit the groove in the valve.
U.S. Pat. No. 1,479,506 to Kelleman for Screw Driver discloses a screwdriver having concave, hollow ground, oppositely facing tip surfaces. Oppositely facing planar surfaces extend from the tip surfaces toward the handle with a tapering to the nominal thickness of the screwdriver shaft connected to the handle.
U.S. Pat. No. 2,684,094 to Lissy for Nonslip Screw Driver and Screwhead discloses a screw head having undercut walls and a screw driver having outwardly flared projections for increasing the grip with the screw.
U.S. Pat. No. 2,782,823 to Williams, Jr., discloses a Magnetic Screw Driver including a bit received by a holder that also receives a permanent magnet capable of magnetizing the bit.
U.S. Pat. No. 2,792,039 to Wing et al for Slotted Screw Head and Driver Therefor Having Non-Burring Engagement discloses a combination screw head and a driver for the screw head where the screwdriver has an arcuate edge and side walls comprising surfaces of revolution diverging from each other toward said edge to engage with undercut portions of the screw slot.
U.S. Pat. No. 2,994,354 to Vaughn for Improved Screw Driver discloses a screwdriver having an arcuate bottom and parallel or converging oppositely facing tip surfaces for use with screw head slots provided with undercut non-planar surfaces.
U.S. Pat. Nos. 3,026,920 and 3,120,251 to York are each entitled Screwdriver and disclose screwdrivers having convex arcuate tips and concave opposite bearing surfaces defined by generally planar surface portions.
U.S. Pat. Nos. 3,897,812 and 3,923,088 to Arnn for Screw Driver and Biting Screw Driver each disclose a screwdriver having a foot portion shaped as an isosceles trapezoid in cross section to increase the gripping force with a screw slot.
U.S. Pat. No. 4,311,071 to Bassell for Screw Driver and Screw Head System discloses in combination a screw head and screw driver having oppositely facing diverging tip faces for engagement with the undercut side walls of the screw slot.
U.S. Pat. No. 4,452,289 to Smith discloses a Combination Hand Grip and Bits Storage for a screwdriver.